1. Field of the Invention
The present invention relates to vertical screw conveyors for vertical lifting of loose granular materials, and more particularly to open helix type flexible tube conveyors without a central shaft that are fitted at their distal ends with a rotating conical scoop with symmetrically distributed radial scoops around its working face.
2. Background
Conventional helix conveyors, screw conveyors, spiral conveyors, auger conveyors, and other flexible screw conveyors typically use a rotating spring steel or stainless-steel flexible screw inside a stationary pipe. The flexible screw inside is driven by an electric motor normally located at a discharge end of such conveyors. This simple design offers advantages over rigid screw conveyors, bucket elevators, drag chain conveyors, aero mechanical conveyors and other equipment intended to transport powder and bulk solid materials under certain conditions and for many materials. But they do have trouble moving, and especially initially feeding, materials vertically straight up.
The trouble starts at the distal end. The material has to be quickly accelerated up to speed high enough for the centrifugal force against the outer wall to create enough friction to keep it from falling, back down. Most commercial open-coil helical screw conveyor systems sold today tilt the whole conveyor, or at least the distal end, to the side to help the distal-end intake and to pack the material into the layer on the wall better.
The intake end of a flexible screw conveyor tube is typically equipped with a charging adapter, a U-shaped trough that connects to the outlet of a hopper or directly to the outlet of process equipment such as a bulk bag discharger, bag dump station, grinder, crusher, screener, blender, reactor or storage vessel of any kind.
Conventional “flexible” conveyors usually have diameters in the two- to four-inch range, and rotate at several hundred RPM. Fast enough that the G-levels developed inside from centrifugal force rises to three to four G's, that is if they are to function well. Even at 2-G's such would barely function. Too much developed centrifugal force causes other problems with stiction. Some suppliers of such conveyors mention having flexible pipes with a maximum diameter of 8-inches. Of course, with lower rotational speeds to keep the centrifugal forces developed inside at optimal and functional levels.
The typical outer casings are usually a very thick plastic tube that can be bent slightly, but are typically quite stationary in use. They are not usually moved around and flexed in different directions. Such bends enables the lifting feeder/conveyor to be positioned in a near vertical orientation, after the tilted entrance region, e.g., to save floor space. If these could be fed effectively at the steeper inclinations that approach vertical, they would probably not need to be bent at all. Anyway, bends have a cost in higher wall loads and wear, and provide very little benefit in return.
Large holds in modern cargo ships are used worldwide to store and transport dry granular materials like grains, cement, ores, and plastics. Pouring these materials into the cargo holds is easy enough, but special dry granular material vertical conveyors are needed to extract them. Specialized ship-unloading screw conveyors with central shafts and vertically oriented are fed by counter rotating scoops that force-feed the lifting feeder/conveyor. These can be get as large as 24-inches in diameter, and can move 2400-tons of bulk material every hour out of the hold of a ship. Siwertell (Cargotel Company, Sweden) is one maker of those big conveyors, and they are in a class by themselves.
In the Siwertell-type vertical screw conveyor a separately powered outer pipe surrounds a stationary conveying tube. The outer pipe has scoops at the bottom that feed material into the conveying screw. The stationary conveying tube between the open-helical screw 108 and the outer scoops stops one flight short of the bottom end, and counter rotating scoops bring material into the reach of the open-helical screw.
Not so different in construction from the conveyors described here are devices proposed for use in extraterrestrial missions that plow into regolith and pull up grains and powders of the regolith. These require a special plow-head with grater-like scoops and ways to move the materials in reduced gravity environments.
Conventional vertical conveyors to plow and lift dry granular materials generally need some horizontal tilt at their plow-tip ends, or need to be deeply submerged in a granular material, to get the material to fill inside the screw and move up to the surface. We describe an improvement of such a mechanism in our U.S. Pat. No. 9,334,693, issued May 10, 2016. Its flexible auger screw uses a central driveshaft in the main pipe and both external scoops and internal lifts radially arranged around a plow-head nose shell.
Another practical problem in reduced-gravity environments is that cohesive materials can sometimes be a problem to transport from a collection site to a collecting and processing site. Granular solids cannot simply be pushed along inside a cylinder with a piston, because frictional loads in a granular slug pushed along a pipe increase exponentially with the length of the slug. So screw conveyors have been conventionally used for transporting free-flowing granular solids over modest distances.
The present inventor, Otis Walton, describes a few centrifuging conveyors for moving granular solid materials over a wide range of cohesion strengths, in U.S. Pat. No. 8,607,966, which issued Dec. 17, 2013. A screw auger is fixed to the outside surface of a fixed inner shaft. A matching, but rotating outer pipe is slipped over the outer diameter of the auger screw. The outer pipe is rotated at a high enough rate to induce granular materials introduced at an input end to cling to the outer pipe's interior walls. The auger screw will act on these clinging layers to move the granular material along to an outlet end, without clumping or clogging.
These centrifuging conveyors need feeders that can introduce granular solid materials at an input end that are boosted in speed enough to have the necessary centrifugal forces come into action. Vertical and steeply inclined orientations need specially adapted scoop-type feeders, as illustrated by FIG. 4B in U.S. Pat. No. 8,607,966. Granular solid material is scooped into bullnose rotating scoop 102 409 externally louvered scoops 410. Axially tapered and tilted inner blades 411 ramp up and boost the introduced materials to near the speed of rotation of bullnose rotating scoop 102 409 to start the materials moving along toward the outlet end in the layers clinging to the interior walls.
The outer pipe rotates on its axis at a rate high enough for even cohesive materials to form in layers on the interior walls. Curved auger screw blades that are either stationary or rotated at a different rate move the materials along inside. As the incoming materials are fed in, a difference in the rotational rates of the interior walls and the set angles of the several blades will dictate how fast the cohesive material is moved along the walls of the interior.
The present invention improves over conventional screw-auger conveyors with stationary outer pipes. Conventional screw-auger conveyors rotate only an inner helical screw inside of the stationary pipe. Conventional screw augers typically operate in one of two modes, in orientations where the axis is near horizontal they typically operate with slow rotation of the inner screw, and they function by material moving up the rising face of the rotating helical screw and then sliding back down its advancing face-inducing axial displacement. These slow-rotating augers, whether they are open-coil augers, or central-shaft screw-augers, will stop working if the screw axis is tilted up to steep angles, or in a near-vertical orientation. Near-horizontal auger conveying, at these low rotation rates, depends on gravity to help convey dry granular solids.
Conventional screw-augers can, however, be operated in such a manner that they convey material vertically, but in this case higher rotation rates are involved and the flow modes and mechanisms that move the materials along inside up the length are significantly different from those operating in slowly rotated screw conveyors. Gravity is no longer helping and must be overcome instead.
In this vertical conveying mode the fast-rotating screws fling the material out to the outer interior walls where friction keeps it from sliding back down, and it can be pushed up along the wall by the helical screw auger as it rotates. The friction of the material moving in a spiral upward path along the outer wall also minimizes the amount of material that falls back down any gap that exists between the open-helical screw 108 and wall. High rotation rates are needed to create the high centrifugal force required for this mode of near-vertical screw-conveying to function. However high rotational rates create problems at the bottom inlet feeds.
A majority of heavy industrial vertical screw conveyor are sold with attached, but separately powered, horizontal feed augers to force-feed the inlet. Other vertical conveying systems have scoops on separately rotating outer pipes to force materials into the conveying systems. Feeding vertically oriented conventional screw-auger conveyors is difficult, and few satisfactory solutions have been developed other than the separate horizontal feed augers.
Some attempts to get good enough inlet feeds for vertical screw conveyors have a retracted outer casing at the distal end that exposes several of the open-helical screw 108 flights. This arrangement can act as a feeder. But screw extensions out beyond the end of the casing can actually fling material out and away from the open-helical screw 108 entry. The centrifugal effects in the swirling material are responsible for feed-starving that gets worse if the rotation rates increase, or gravity is reduced.
Vertical conveying tests under lunar gravity conditions aboard NASA's reduced gravity aircraft with lunar stimulants proved fast rotating screws would feed-starve and not be very effective.
There is a general need therefore for a device that can quickly feed enough material into vertical screw-auger conveying systems. And in the field of space exploration and in-situ resource utilization, there is a need for devices to extract subsurface materials through small entry holes into the surface of the moon or Mars, e.g., to preserve volatiles from Space. A plow-head feeder is needed for connection to flexible conveying systems when excavating materials. And one that can convey the materials to the surface through a small sealed inlet hole, and thus meet the needs of future space exploration missions.